Still, if you're a western monk, why stay in Asia? You might be more beneficial to people in the west.
Monks with Asian accents and bad English, while western people find them alluring, are not always as easily understood. And there are people like Gil Fronsdal, who are so brilliant, but they don't describe things with a rigorous adherence to the Pali text, or have any monastic authority, so people don't take them as seriously. Also, it seems like there are so many Buddhist temples in America that only (or mostly only) cater to Asian groups. But there is already a plethora of monastics in places like Thailand, isn't there?
Because of visa issues, most westerners at some point in time would need to return home. I get the impression that many western monks, other than the monks associated with the ajahn Chan monasteries and particularly those ordained in Sri Lanka or Burma often disrobe after some time in the west. They don't feel comfortable in ethnic monasteries and are unable to get sufficient support for western monasteries.